Preparation of permanent magnet

Abstract: A permanent magnet which contains R, T and B as main ingredients wherein R is Y or a rare earth element and T is Fe or Fe and Co and has a primary phase of R 2 T 14 B is produced by compacting a mixture of 60 to 95 wt% of a primary phase-forming master alloy and a grain boundary phase-forming master alloy both in powder form and sintering the compact. The primary phase-forming master alloy has columnar crystal grains of R 2 T 14 B with a mean grain size of 3-50 µm and grain boundaries of an R rich phase and contains 26-32 wt% of R. The grain boundary phase-forming master alloy is a crystalline alloy consisting essentially of 32-60 wt% of R and the balance of Co or Co and Fe. In anther form, a permanent magnet which contains R, T and B as main ingredients wherein R is yttrium or a rare earth element, T is Fe or Fe + Co/Ni and has a primary phase of R 2 T 14 B is produced by compacting a mixture of a primary phase-forming master alloy and a grain boundary-forming master alloy both in powder form and sintering the compact. The primary phase-forming master alloy has a primary phase of R 2 T 14 B and grain boundaries of an R rich phase. The grain boundary-forming master alloy contains 40-65 wt% of R, 30-60 wt% of Fe, Co or Ni and 1-12 wt% of Sn, In or Ga.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a method for preparing rare earth permanent magnets. Prior Art Rare earth magnets of high performance, typically powder metallurgical Sm-Co base magnets having an energy product of 32 MGOe have been produced on a large commercial scale. However, these magnets suffer from a problem that the raw materials, Sm and Co, cost much. Of rare earth elements, some elements of low atomic weight, e.g., Ce, Pr, and Nd are available in more plenty and less expensive than Sm. Iron is less expensive than cobalt. For these reasons, R-T-B base magnets (wherein R stands for a rare earth element and T stands for Fe or Fe plus Co) such as Nd-Fe-B and Nd-Fe-Co-B magnets were recently developed. One example is a sintered magnet as set forth in Japanese Patent Application Kokai (JP-A) No. 59-46008. Sintered magnets may be produced by applying a conventional powder metallurgical process for Sm-Co systems (melting → master alloy ingot casting → ingot crushing → fine pulverization compacting sintering magnet), and excellent magnetic properties are readily available. Generally, a master alloy ingot produced by casting has a structure wherein crystal grains made up of a ferromagnetic R 2 Fe 14 B phase (referred to as a primary phase, hereinafter) are covered with a non-magnetic R-rich phase (referred to as a grain boundary phase, hereinafter). The master alloy ingot is then pulverized or otherwise reduced to a particle diameter smaller than the crystal grain diameter, offering a magnet powder. The grain boundary phase has a function to promote sintering by converting into a liquid phase and plays an important role for the sintered magnet to generate coercivity. One typical method for the preparation of R-T-B sintered magnets is known as a two alloy route. The two alloy route is by mixing two alloy powders of different compositions and sintering the mixture, thereby improving magnetic properties and corrosion resistance. A variety of proposals have been made on the two alloy route. All these proposals use an alloy powder having approximately the same composition (R 2 T 14 B) as the primary phase of the final magnet and add a subordinate alloy powder thereto. The known subordinate alloys used heretofore include R rich alloys having a higher R content and a lower melting point than the primary phase (JP-A 4-338607 and USP 5,281,250 or JP-A 5-105915), R 2 T 14 B alloys containing a different type of R from the primary phase (JP-A 61-81603), and alloys containing an intermetallic compound of R (JP-A 5-21219). One of the alloys used in these two alloy methods is a primary alloy of the composition R 2 T 14 B. If the primary alloy is produced by a melt casting process, a soft magnetic α-Fe phase precipitates to adversely affect high magnetic properties. It is then necessary to carry out solution treatment, typically at about 900°C or higher for one hour or longer. In JP-A 5-21219, for example, an R 2 T 14 B alloy prepared by a high-frequency melting process is subject to solution treatment at 1,070°C for 20 hours. Because of such a need for high temperature, long time solution treatment, the melt casting method is against low cost manufacture. USP 5,281,250 produces an R 2 T 14 B alloy by a direct reduction and diffusion process, which alloy has an isometric crystal system and poor magnetic properties. A higher calcium content also precludes manufacture of high performance magnets. JP-A 4-338607 uses a crystalline or amorphous R 2 T 14 B alloy powder which is produced by a single roll process so as to have microcrystalline grains of up to 10 µ m. It is not described that the grains are columnar. It is rather presumed that the grains are isometric because magnetic properties are low. JP-A 4-338607 describes that the grain size is limited to 10 µ m or less in order to prevent precipitation of soft magnetic phases such as α-Fe. With respect to thermal stability, R-T-B magnets are less stable than the Sm-Co magnets. For example, the R-T-B magnets have a differential coercivity iHc/ T as great as -0.60 to -0.55%/°C in the range between room temperature and 180°C and undergo a significant, irreversible demagnetization upon exposure to elevated temperatures. Therefore, the R-T-B magnets are rather impractical when it is desired to apply them to equipment intended for high temperature environment service, for example, electric and electronic devices in automobiles. For reducing the irreversible demagnetization upon heating of R-T-B magnets, JP-A 62-165305 proposes to substitute Dy for part of Nd and Co for part of Fe. However, it is impossible to achieve a substantial reduction of iHc/ T by merely adding Dy and Co. Larger amounts of Dy substituted sacrifice maximum energy product (BH)max. JP-A 64-7503 proposes to improve thermal stability by adding gallium (Ga) while IEEE Trans. Magn. MAG-26 (1990), 1960 proposes to improve thermal stability by adding molybdenum (Mo) and vanadium (V). The addition of Ga, Mo and V is effective for improving thermal stability, but sacrifices maximum energy product. DE-A-4135403 discloses magnets having a boron-free phase of formula SE 6 Fe 13 M in addition to the main SE 2 Fe 14 B phase, resulting in a rise in coercive force and an improvement in temperature dependence. We proposed to add tin (Sn) and aluminum (Al) for improving thermal stability with a minimal loss of maximum energy product (JP-A 3-236202). Since the addition of Sn, however, still has a tendency of lowering maximum energy product, the amount of Sn added should desirably be limited to a minimal effective level. German Patent Application DE-A-4027598 discloses magnets that, in addition to the main SE 2 Fe 14 B phase, have a phase where tin accumulates of formula SE 6 Fe 13 Sn. However, this application does not disclose the addition of an SE 6 Fe 13 Sn-containing alloy. It was also reported to add tin (Sn) to magnets using a so-called two alloy route. The two alloy route is by mixing two alloy powders of different compositions, typically an alloy powder having a composition approximate to the primary phase composition and a subordinate alloy powder having a composition approximate to the grain boundary phase composition and sintering the mixture. For instance, Proc. 11th Inter. Workshop on Rare-Earth Magnets and their Applications, Pittsburgh, 1990, p. 313 discloses that a sintered magnet is prepared by mixing Nd 14.5 D y1.5 Fe 75 AlB 8 alloy powder with up to 2.5% by weight of Fe 2 Sn or CoSn powder, followed by sintering. It is reported that this sintered magnet has a Nd 6 Fe 13 Sn phase precipitated in the grain boundary phase and is improved in thermal dependency of coercivity. Making a follow-up experiment, we found that the Fe 2 Sn or CoSn material is unlikely to fracture and thus difficult to comminute into a microparticulate powder having a consistent particle size. Then sintered magnets resulting from a mixture of an R-T-B alloy powder and a Fe 2 Sn or CoSn powder contain unevenly distributed Nd 6 Fe 13 Sn phase of varying size. This is also evident from Figure 5 of the above-referred article. It is thus difficult to provide thermal stability in a consistent manner. where tin is added in the form of Fe 2 Sn or CoSn powder, R and Fe in the primary phase are consumed to form Nd 6 Fe 13 Sn, which can alter the composition of the primary phase, deteriorating magnetic properties. SUMMARY OF THE INVENTION The present invention is defined in claim 1, preferred embodiments of the invention are defined in the dependent claims 2 to 18. According to the invention, a sintered rare earth magnet is produced by a so-called two alloy route. The two alloy route for producing a sintered rare earth magnet involves compacting a mixture of a primary phase-forming master alloy and a grain boundary phase-forming master alloy both in powder form and sintering the compact. In the present invention, there is provided a method for preparing a permanent magnet which contains R, T and B as main ingredients and has a primary phase consisting essentially of R 2 T 14 B. Herein R is at least one element selected from the group consisting of yttrium and rare earth elements, T is iron or a mixture of iron and cobalt, and B is boron. The method involves the steps of compacting a mixture of a primary phase-forming master alloy and a grain boundary-forming master alloy both in powder form and sintering the compact. In one preferred embodiment, the primary phase-forming master alloy is produced by cooling an alloy melt from one direction or two opposite directions by a single roll, twin roll or rotary disk process; the primary phase-forming master alloy as cooled has a thickness of 0.1 to 2 mm in the cooling direction; the primary phase-forming master alloy is substantially free of an α-Fe phase. The primary phase-forming master alloy has a primary phase consisting essentially of R 2 T 14 B and grain boundaries composed mainly of an R rich phase having a higher R content than R 2 T 14 B. The grain boundary-forming master alloy contains 40 to 65% by weight of R, 30 to 60% by weight of T' and 1 to 12% by weight of M. Herein T' is at least one element selected from iron and cobalt and M is at least one element selected from the group consisting of tin, indium and gallium. Preferably M contains 30 to 100% by weight of tin. Preferably the permanent magnet consists essentially of 27 to 38% by weight of R, 0.5 to 4.5% by weight of B, 0.03 to 0.5% by weight of M, and 51 to 72% by weight of T. Preferably the permanent magnet contains an R 6 T' 13 M phase in the grain boundary. Preferably the mixture contains 99.2 to 90% by weight of the primary phase-forming master alloy and 0.2 to 10% by weight of the grain boundary-forming master alloy. Preferably the grain boundary-forming master alloy has an R 6 T' 13 M phase. Preferably the primary phase of the primary phase-forming master alloy contains columnar crystal grains having a mean grain size of 3 to 50 µ m. In another preferred embodiment, the grain boundary phase-forming master alloy contains grains having a mean grain size of up to 20 µ m; the grain boundary phase-forming master alloy is produced by cooling an alloy melt from one direction or two opposite directions by a single roll, twin roll or rotary disk process; and the grain boundary phase-forming master alloy as cooled has a thickness of 0.1 to 2 mm in the cooling direction. In a further preferred embodiment, the primary phase-forming master alloy in powder form is produced by causing the alloy to occlude hydrogen and pulverizing the alloy by a jet mill; the grain boundary phase-forming master alloy in powder form is produced by causing the alloy to occlude hydrogen and pulverizing the alloy by a jet mill; and the alloys are heated to a temperature of 300 to 600°C, subjected to hydrogen occlusion treatment, and then pulverized without hydrogen release. The hydrogen occlusion may be optionally followed by hydrogen release. The present invention seeks to provide the following advantages. Regarding magnets prepared by sintering an R-T-B system alloy powder with Sn added thereto, we have found that the sintered magnets contain R 6 T 13 Sn at the grain boundary, this R 6 T 13 Sn created at the grain boundary is effective for improving thermal stability, and a tin residue in the primary phase contributes to a lowering of maximum energy product. Accordingly, for the purpose of adding M to an R-T-B system magnet wherein M is at least one of Sn, In, and Ga, the present invention adopts a two alloy route and employs an M-containing alloy as the grain boundary-forming master alloy rather than adding M to the primary phase-forming master alloy. Since M is added to only the grain boundary-forming master alloy, satisfactory thermal stabilization is accomplished with minor amounts of M. The present invention uses as the grain boundary-forming master alloy an alloy having a composition centering at R 6 T' 13 M wherein T' is at least one of Fe and Co. Unlike the Fe 2 Sn and CoSn alloys, the alloy of this composition is easy to pulverize so that it can be readily comminuted into a microparticulate powder, especially with the aid of hydrogen occlusion. As a consequence, the sintered magnet contains evenly distributed R 6 T' 13 M phase of consistent size in the grain boundary. It is then possible to produce thermally stable magnets on a mass scale. In contrast, the aforementioned Fe 2 Sn and CoSn alloys are not fully milled even with the aid of hydrogen occlusion since little hydrogen can be incorporated therein. The use of an alloy having a composition centering at R 6 T' 13 M as the grain boundary-forming master alloy allows the R 6 T' 13 M phase to form in the grain boundary without substantial influence on the primary phase composition. This permits the magnet to exhibit magnetic properties inherent to the composition of the primary phase-forming master alloy without a loss. When the grain boundary-forming master alloy has a grain size within the range, a finer powder is obtained, which ensures that the sintered magnet contains more evenly distributed R 6 T' 13 M phase of more consistent size. Then the magnet has higher magnetic properties and higher thermal stability thereof. The grain boundary-forming master alloy having such a grain size can be prepared by a single or twin roll process, that is, by cooling an alloy melt from one direction or two opposite directions. In general, the two alloy route uses an alloy having a composition approximate to R 2 T 14 B as the primary phase-forming master alloy. If this alloy is prepared by a melt casting process, a magnetically soft α-Fe phase would precipitate to adversely affect magnetic properties. A solution treatment is then required. The solution treatment should be carried out at 900°C or higher for one hour or longer. In JP-A 5-21219, for example, an R 2 T 14 B alloy obtained by high-frequency induction melting is subject. to solution treatment at 1,070°C for 20 hours. Due to a need for such high temperature, long term solution treatment, magnets cannot be manufactured at low cost with the melt casting process. If an R 2 Fe 14 B alloy to be used in the twc alloy route is prepared by a direct reduction and diffusion process as disclosed in JP-A 5-105915, the alloy has a too increased calcium content for magnets to have satisfactory properties. In contrast, the preferred embodiment of the invention uses a primary phase-forming master alloy containing columnar grains having a mean grain size of 3 to 50 µ m. This alloy has an R rich phase uniformly dispersed and is substantially free of an α-Fe phase. As a result, the magnet powder obtained by finely dividing the primary phase-forming master alloy has a minimal content of magnet particles free of the R rich phase, with substantially all magnet particles having an approximately equal content of the R rich phase. Then the powder can be effectively sintered and the dispersion of the R rich phase is well maintained during sintering so that high coercivity is expectable. Also the master alloy can be pulverized in a very simple manner to provide a sharp particle size distribution which ensures a sufficient distribution of crystal grain size after sintering to develop high coercivity. A brief pulverization time reduces the amount of oxygen entrained, achieving a high residual magnetic flux density. The particle size distribution becomes very sharp particularly when hydrogen occlusion assists in pulverization. The invention eliminates a need for solution treatment for extinguishing an α-Fe phase. Like the grain boundary-forming master alloy, the primary phase-forming master alloy can be prepared by a single or twin roll process, that is, by cooling an alloy melt from one direction or two opposite directions. The above-referred JP-A 4-338607 discloses that a crystalline or amorphous RE 2 T 14 B 1 alloy powder having a fine grain size of up to 10 µ m and a RE-T alloy are produced by a single roll process. However, no reference is made to the thickness of the alloy in the cooling direction and the grain size of the RE-T alloy. The RE-T alloy used therein has a composition different from the grain boundary-forming master alloy used in the present invention. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, the following description is made in conjunction with the accompanying drawings. FIG. 1 is a partly cut-away, side view of a jet mill utilizing a fluidized bed. FIG. 2 illustrates a portion of a jet mill utilizing a vortex flow, FIG. 2a being a horizontal cross section and FIG. 2b being an elevational cross section. FIG. 3 is a cross-sectional view showing a portion of a jet mill utilizing an impingement plate. FIG. 4 is a photograph showing the columnar grain structure appearing in a section of a master alloy produced by a single roll technique. DETAILED DESCRIPTION OF THE INVENTION According to the present invention, a sintered rare earth magnet is prepared by compacting a mixture of a primary phase-forming master alloy and a grain boundary phase-forming master alloy both in powder form and sintering the compact. Primary phase-forming master alloy The primary phase-forming master alloy contains R, T and B as main ingredients wherein R is at least one element selected from the group consisting of yttrium (Y) and rare earth elements, T is iron or a mixture of iron and cobalt, and B is boron. The alloy has a phase consisting essentially of R 2 T 14 B and grain boundaries composed mainly of an R rich phase having a higher R content than R 2 T 14 B. The rare earth elements include lanthanides and actinides. At least one of Nd, Pr, and Tb is preferred, with Nd being especially preferred. Additional inclusion of Dy is preferred. It is also preferred to include at least one of La, Ce, Gd, Er, Ho, Eu, Pm, Tm, Yb, and Y. Mixtures of rare earth elements such as misch metal are exemplary sources. The composition of the primary phase-forming master alloy is not critical insofar as the above-mentioned requirements are met. A particular composition of the master alloy may be suitably determined in accordance with the target magnet composition while considering the composition of the grain boundary phase-forming master alloy and its mixing proportion. Preferably the primary phase-forming master alloy consists essentially of 27 to 38% by weight of R, 0.9 to 2% by weight of B, and the balance of T. A boron content of less than 0.9% by weight fails to provide high coercivity whereas a boron content of more than 2% by weight fails to provide high residual magnetic flux density. Additionally, an element selected from Al, Cr, Mn, Mg, Si, Cu, C, Nb, W, V, Zr, Ti, and Mo may be added. A residual magnetic flux density will lower if the amount of such an additive element exceeds 6% by weight. In addition, the primary phase-forming master alloy may further contain incidental impurities or trace additives such as carbon and oxygen. Preferably the primary phase of the primary phase-forming master alloy contains columnar crystal grains having a mean grain size of 3 to 50 µ m, more preferably 5 to 50 µ m, further preferably 5 to 30 µ m, most preferably 5 to 15 µ m. If the mean grain size is too small, magnet particles obtained by pulverizing the alloy would be polycrystalline and fail to achieve a high degree of orientation. If the mean grain size is too large, the advantages of the invention would not be fully achieved. It is to be noted that the mean grain size of columnar grains is determined by first cutting or polishing the master alloy to expose a section substantially parallel to the major axis direction of columnar grains, and measuring the width in a transverse direction of at least one hundred columnar grains in this section. The width measurements are averaged to give the mean grain size of columnar grains. The columnar grains have an aspect ratio (defined as a major axis length to width ratio) which is preferably between about 2 and about 50, especially between about 5 and about 30 though not limited thereto. The primary phase-forming master alloy has a good dispersion of an R rich phase, which can be observed in an electron microscope photograph (or reflection electron image). The grain boundary composed mainly of the R rich phase usually has a width of about 0.5 to 5 µ m in a transverse direction although the width varies with the R content. Preferably, the primary phase-forming master alloy having such a structure is produced by cooling an alloy melt containing R, T and B as main ingredients from one or two opposite directions. The thus produced master alloy has columnar grains arranged such that their major axis is oriented in substantial alignment with the cooling direction. The term "cooling direction" used herein refers to a direction perpendicular to the surface of a cooling medium such as the circumferential surface of a chill roll, i.e., a heat transfer direction. For cooling the alloy melt in one direction, single roll and rotary disk techniques are preferably used. The single roll technique is by injecting an alloy melt through a nozzle toward a chill roll for cooling by contact with the peripheral surface thereof. The apparatus used therein has a simple structure and a long service life and is easy to control the cooling rate. A primary phase-forming master alloy usually takes a thin ribbon form when produced by the single roll technique. Various conditions for the single roll technique are not critical. Although the conditions can be suitably determined such that the primary phase-forming master alloy having a structure as mentioned above may be obtained, the following conditions are usually employed. The chill roll, for instance, may be made of various materials that are used for conventional melt cooling procedures, such as Cu and Cu alloys (e.g., Cu-Be alloys). An alternative chill roll is a cylindrical base of a material as mentioned just above which is covered with a surface layer of a metal material different from the base material. This surface layer is often provided for thermal conductivity control and wear resistance enhancement. For instance, when the cylindrical base is made of Cu or a Cu alloy and the surface layer is made of Cr, the primary phase-forming master alloy experiences a minimal differential cooling rate in its cooling direction, resulting in a more homogeneous master alloy. In addition, the wear resistance of Cr ensures that a larger quantity of master alloy is continuously produced with a minimal variation of properties. The rotary disk technique is by injecting an alloy melt through a nozzle against a rotating chill disk for cooling by contact with the surface thereof. A primary phase-forming master alloy is generally available in scale or flake form when produced by the rotary disk technique. It is noted, however, that as compared with the single roll technique, the rotary disk technique involves some difficulty in achieving uniform cooling rates because master alloy flakes are more rapidly cooled at the periphery than the rest. A twin roll technique is effective for cooling an alloy melt from two opposite directions. This technique uses two chill rolls, each being similar to that used in the single roll technique, with their peripheral surfaces opposed to each other. The alloy melt is injected between the opposed peripheral surfaces of the rotating rolls. A primary phase-forming master alloy is generally available in a thin ribbon or thin piece form when produced by the twin roll technique. Various conditions for the twin roll technique are not critical, and can be suitably determined such that the above-mentioned structure may be obtained. Most preferred among these cooling techniques is the single roll technique. It is understood that the alloy melt is preferably cooled in a non-oxidizing atmosphere such as nitrogen and argon or in vacuum. When a primary phase-forming master alloy is produced by cooling an alloy melt from one or two opposite directions, it preferably has a thickness of 0.1 to 2 mm, more preferably 0.2 to 1.0 mm and most preferably 0.2 to 0.5 mm as measured in the cooling direction. With a thickness of less than 0.1 mm, it would be difficult to obtain columnar grains having a mean grain size of more than 3 µ m. With a thickness exceeding 2 mm, the resulting structure would become more uneven in the cooling direction particularly when cooled from one direction. More particularly, since grains are sized too small on the cooling side, the alloy tends to form polycrystalline particles when pulverized, which would degrade sintered density and orientation, failing to provide satisfactory magnetic properties. With a too much thickness in the cooling direction, it would also be difficult to obtain columnar grains having a mean grain size of less than 50 µ m. In this sense, the twin roll technique is effective for suppressing excess grain growth. When the melt is cooled in one or two directions, the columnar grains have a length coincident with the thickness of a thin ribbon or piece. The structure of the thin ribbon or piece consists essentially of columnar grains while isometric grains, if any, can exist only as chilled grains at the cooling surface and in an amount of less than 10%, especially 5% by volume as observed under SEM. With such a cooling technique used, a primary phase-forming master alloy that is substantially free of an α-Fe phase can be produced even when the starting composition has a relatively low R content, for instance, an R content of about 26 to 32% by weight. More particularly, the content of α-Fe phase can be reduced to less than 5% by volume, especially less than 2% by volume. This eliminates a solution treatment for reducing the proportion of distinct phases. Grain boundary phase-forming master alloy The grain boundary phase-forming master alloy contains R, T' and M wherein R is as defined above, T' is at least one element selected from the group consisting of iron (Fe) and cobalt (Co), and M is at least one element selected from the group consisting of tin (Sn), indium (In) and gallium (Ga), wherein M contains 30 to 100% by weight of tin (Sn). The master alloy consists essentially of 40 to 65% by weight of R, 30 to 60% by weight of T', and 1 to 12% by weight of M, preferably 50 to 60% by weight of R, 40 to 50% by weight of T', and 4 to 10% by weight of M. A master alloy with a much higher R content is oxidizable and thus unsuitable as a starting source material. With a much higher T' content, magnetically soft distinct phases such as α-Fe precipitate to deteriorate magnetic properties. With a too lower R or T' content, formation of an R 6 T' 13 M phase during sintering, which will be described later, alters the composition of the primary phase to deteriorate magnetic properties. The composition of the R component in the grain boundary-forming master alloy (that is, the proportion of yttrium and rare earth elements in the R component) is not particularly limited although it is preferably substantially the same as the composition of the R component in the primary phase-forming master alloy because it is then easy to control the final magnet composition. Cobalt is effective for improving the corrosion resistance of a magnet, but functions to lower the coercivity if it is contained in the primary phase of the magnet. For a sintered magnet, it is then preferred that cobalt be contained mainly in the grain boundary phase of the magnet. For this reason, cobalt is contained in the grain boundary phase-forming master alloy according to the present invention. Additional elements such as Al, Si, Cu, Nb, W, V and Mo may be added to the grain boundary phase-forming master alloy in an amount of up to 5% by weight for suppressing a substantial loss of residual magnetic flux density. In addition, the grain boundary phase-forming master alloy may further contain incidental impurities or trace additives such as carbon and oxygen. The grain boundary phase-forming master alloy, when it is crystalline, generally comprises a mix phase which contains at least one of R 6 T' 13 M, RT' 2 , RT' 3 , RT' 7 , and R 5 T' 13 phases and may additionally contain any of other R-T' and R-T'-M phases. This does not depend on a preparation method. The R 6 T' 13 M phase is of a body centered cubic system. The presence of respective phases can be confirmed by electron radiation diffractometry, for example, as described in J. Magnetism and Magnetic Materials, 101 (1991), 417-418. In general, a plurality of phases as mentioned above are contained in the crystalline grain boundary-forming master alloy which is prepared by an arc melting method, high-frequency induction melting method, or rapid quenching method such as a single roll technique. The alloy is pulverized as such according to the present invention while it may be annealed for increasing the proportion of R 6 T' 13 M phase or creating a R 6 T' 13 M phase. This annealing may be effected at a temperature of about 600 to 900°C for about 1 to 20 hours. Too high annealing temperatures would cause Nd to be dissolved whereas too low annealing temperatures would induce little change of the phase structure. Preferably the grain boundary phase-forming master alloy contains columnar crystal grains having a mean grain size of up to 20 µ m, more preferably up to 10 µ m. With a too large mean grain size of more than 20 µ m, the distribution of the above-mentioned phases would be non-uniform. Then the alloy is pulverized into particles which would have largely varying compositions. If a grain boundary phase-forming master alloy powder comprising such variable composition particles is mixed with a primary phase-forming master alloy powder, the composition would become non-uniform and precipitation of a R 6 T' 13 M phase playing an important role in improving properties would be hindered. Additionally there would occur a region where the primary phase composition is altered by precipitation of a R 6 T' 13 M phase, resulting in insufficient thermal stability and magnetic properties (coercivity and squareness ratio). The lower limit of the mean grain size is not specified. This means that an amorphous grain boundary-forming master alloy is acceptable. It is understood that if the mean grain size is too small, the alloy becomes too fragile so that a large amount of ultra-fine debris is generated upon pulverization. Such ultra-fine debris is difficult to recover. When a mixture of the two master alloys in crude powder form is finely milled, the percentage recovery of the grain boundary phase-forming master alloy is selectively reduced or varied. This would result in a shift of composition (a lowering of R or M content) and a variation thereof, which in turn, results in a lowering of thermal stability, coercivity and sintered density and a variation thereof. Therefore, the mean grain size may desirably be more than 0.1 µ m, especially more than 0.5 µ m depending on the pulverizing conditions. The grain boundary phase-forming master alloy may be produced by any desired method, for example, a conventional casting method. Preferably it is again produced by cooling an alloy melt from one direction or two opposite directions in the same manner as previously described for the primary phase-forming master alloy. Preferred conditions for such cooling techniques are the same as previously described for the primary phase-forming master alloy. The grain boundary phase-forming master alloy has a thickness in the cooling direction which falls in the same range as previously described for the primary phase-forming master alloy. Pulverization and mixing steps It is not critical how to produce a mixture of a primary phase-forming master alloy powder and a grain boundary phase-forming master alloy powder. Such a mixture is obtained, for example, by mixing the two master alloys, crushing the alloys at the same time, and finely milling the alloys. Alternatively, a mixture is obtained by crushing the two master alloys separately, mixing the crushed alloys, and finely milling the mixture. A further alternative is by crushing and then finely milling the two master alloys separately, and mixing the milled alloys. The last-mentioned procedure of milling the two master alloys separately before mixing is difficult to reduce the cost because of complexity. Where the grain boundary phase-forming master alloy is one produced by a single roll technique and having a small mean grain size, it is preferred to mix the two master alloys and to crush and then mill the alloys together because a uniform mixture is readily available. In contrast, where the grain boundary phase-forming master alloy used is one produced by a melting technique, the preferred procedure is by crushing the two master alloys separately, mixing the crushed alloys, and finely milling the mixture or by crushing and then finely milling the two master alloys separately, and mixing the milled alloys. This is because the grain boundary phase-forming master alloy produced by a melting technique has a so large grain size that crushing the alloy together with the primary phase-forming master alloy is difficult. Preferably the mixture contains 0.2 to 10% by weight, preferably 0.5 to 10% by weight of the grain boundary phase-forming master alloy. The advantages achieved by adding the grain boundary-forming master alloy would be lost if the content of the grain boundary-forming master alloy is too low. Magnetic properties, especially residual magnetic flux density are insufficient if the content is too high. It is not critical how to pulverize the respective master alloys. Suitable pulverization techniques such as mechanical pulverization and hydrogen occlusion-assisted pulverization may be used alone or in combination. The hydrogen occlusion-assisted pulverization technique is preferred because the resulting magnet powder has a sharp particle size distribution. Hydrogen may be occluded or stored directly into the master alloy in thin ribbon or similar form. Alternatively, the master alloy may be crushed, typically to a mean particle size of about 15 to 500 µ m by mechanical crushing means such as a stamp mill before hydrogen occlusion. No special limitation is imposed on the conditions for hydrogen occlusion-assisted pulverization. Any of conventional hydrogen occlusion-assisted pulverization procedures may be used. For instance, hydrogen occlusion and release treatments are carried out at least once for each, and the last hydrogen release is optionally followed by mechanical pulverization. It is also acceptable to heat a master alloy to a temperature in the range of 300 to 600 C, preferably 350 to 450 C, then carry out hydrogen occlusion treatment and finally mechanically pulverize the alloy without any hydrogen release treatment. This procedure can shorten the manufacturing time because the hydrogen release treatment is eliminated. Where the primary phase-forming master alloy is subject to such hydrogen occlusion treatment, there is obtained a powder having a sharp particle size distribution. When the primary phase-forming master alloy is subject to hydrogen occlusion treatment, hydrogen is selectively stored in the R rich phase forming the grain boundaries to increase the volume of the R rich phase to stress the primary phase, which cracks from where it is contiguous to the R rich phase. Such cracks tend to propagate in layer form in a plane perpendicular to the major axis of the columnar grains. Within the primary phase in which little hydrogen is occluded, on the other hand, irregular cracks are unlikely to occur. This prevents the subsequent mechanical pulverization from generating finer and coarser particles, assuring a magnet powder having a uniform particle size. Also the hydrogen occluded within the above-mentioned temperature range forms a dihydride of R in the R rich phase. The R dihydride is fragile enough to avoid generation of coarser particles. If the primary phase-forming master alloy is at a temperature of less than 300 C during hydrogen occlusion, much hydrogen is stored in the primary phase too and, besides, the R of the R rich phase forms a trihydride, which reacts with H 2 O, resulting in a magnet containing much oxygen. If the master alloy stores hydrogen at a temperature higher than 600 C, on the other hand, no R dihydride will then be formed. Conventional hydrogen occlusion-assisted pulverization processes entailed a large quantity of finer debris which had to be removed before sintering. So a problem arose in connection with a difference in the R content of the alloy mixture before and after pulverization. The process of the invention substantially avoids occurrence of finer debris and thus substantially eliminates a shift in the R content before and after pulverization. Since hydrogen is selectively stored in the grain boundary, but little in the primary phase of the primary phase-forming master alloy, the amount of hydrogen consumed can be drastically reduced to about 1/6 of the conventional hydrogen consumption. It is understood that hydrogen is released during sintering of the magnet powder. Also in the hydrogen occlusion treatment of the grain boundary-forming master alloy, hydrogen occlusion causes the alloy to increase its volume and to crack so that the alloy may be readily pulverized. In the practice of the invention, the hydrogen occlusion step is preferably carried out in a hydrogen atmosphere although a mix atmosphere additionally containing an inert gas such as He and Ar or another non-oxidizing gas is acceptable. The partial pressure of hydrogen is usually at about 0.05 to 20 atm., but preferably lies at 1 atm. or below, and the occlusion time is preferably about 1/2 to 5 hours. For mechanical pulverization of the master alloy with hydrogen occluded, a pneumatic type of pulverizer such as a jet mill is preferably used because a magnet powder having a narrow particle size distribution is obtained. The jet mills are generally classified into jet mills utilizing a fluidized bed, a vortex flow, and an impingement plate. FIG. 1 schematically illustrates a fluidized bed jet mill. FIG. 2 schematically illustrates a portion of a vortex flow jet mill. FIG. 3 schematically illustrates a portion of an impingement plate jet mill. The jet mill of the structure shown in FIG. 1 includes a cylindrical vessel 21, a plurality of gas inlet pipes 22 extending into the vessel through the side wall thereof, and a gas inlet pipe 23 extending into the vessel through the bottom thereof wherein gas streams are introduced into the vessel 21 through the inlet pipes 22 and 23. A batch of feed or a master alloy having hydrogen occluded therein is admitted through a feed supply pipe 24 into the vessel 21. The gas streams cooperate with the admitted feed to form a fluidized bed 25 within the vessel 21. The alloy particles collide repeatedly with each other within the fluidized bed 25 and also impinge against the wall of the vessel 21, whereby they are milled or more finely pulverized. The thus milled fine particles are classified through a classifier 26 mounted on the vessel 21 before they are discharged out of the vessel 21. Relatively coarse particles, if any, are fed back to the fluidized bed 25 for further milling. FIGS. 2a and 2b are horizontal and elevational cross-sectional views of the vortex flow jet mill. The jet mill of the structure shown in FIG. 2 includes a bottomed vessel 31 of a generally conical shape, a feed inlet pipe 32 and a plurality of gas inlet pipes 33 extending through the wall of the vessel in proximity to its bottom. Into the vessel 31, a batch of feed is supplied along with a carrier gas through the feed inlet pipe 32, and a gas is injected through the gas inlet pipes 33. The feed inlet pipe 32 and gas inlet pipes 33 are located diagonally and at an angle with respect to the wall of the vessel 31 (as viewed in the plan view of FIG. 2a) so that the gas jets can form a vortex flow in the horizontal plane within the vessel 31 and create a fluidized bed owing to vertical components of kinetic energy. The feed master alloy particles collide repeatedly with each other within the vortex flow and fluidized bed in the vessel 31 and also impinge against the wall of the vessel 31 whereby they are milled or more finely pulverized. The thus milled fine particles are discharged out of the vessel 31 through an upper opening. Relatively coarse particles, if any, are classified within the vessel 31, then sucked into the gas inlet pipes 33 through holes in the side wall thereof, and injected again along with the gas jets into the vessel 31 for repeated pulverization. In the jet mill having the structure shown in FIG. 3, a batch of feed is supplied through a feed hopper 41, accelerated by a gas jet admitted through a nozzle 42, and then impinged against an impingement plate 43 for milling. The milled feed particles are classified, and fine particles are discharged out of the jet mill. Relatively coarse particles, if any, are fed back to the hopper 41 for repeated pulverization in the same manner as mentioned above. It is understood that the gas jets in the jet mill are preferably made of a non-oxidizing gas such as N 2 or Ar gas. The milled particles preferably have a mean particle size of about 1 µ m to about 10 µ m. Since the milling conditions vary with the size and composition of the master alloy, the structure of a jet mill used, and other factors, they may be suitably determined without undue experimentation. It is to be noted that hydrogen occlusion can cause not only cracking, but also disintegration of at least some of the master alloy. When the master alloy after hydrogen occlusion is too large in size, it may be pre-pulverized by another mechanical means before pulverization by a jet mill. Compacting step A mixture of primary phase-forming master alloy powder and grain boundary phase-forming master alloy powder is compacted, typically in a magnetic field. Preferably the magnetic field has a strength of 15 kOe or more and the compacting pressure is on the order of 0.5 to 3 t/cm 2 . Sintering step The compact is fired, typically at 1,000 to 1,200 C for about 1/2 to 5 hours, and then quenched. It is noted that the sintering atmosphere comprises an inert gas such as Ar gas or vacuum. After sintering, the compact is preferably aged in a non-oxidizing atmosphere or in vacuum. To this end two stage aging is preferred. At the first aging stage, the sintered compact is held at a temperature ranging from 700 to 900 C for 1 to 3 hours. This is followed by a first quenching step at which the aged compact is quenched to the range of room temperature to 200 C. At the second aging stage, the quenched compact is retained at a temperature ranging from 500 to 700 C for 1 to 3 hours. This is followed by a second quenching step at which the aged compact is again quenched to room temperature. The first and second quenching steps preferably use a cooling rate of 10 C/min. or higher, especially 10 to 30 C/min. The heating rate to the hold temperature in each aging stage may usually be about 2 to 10 C/min. though not critical. At the end of aging, the sintered body is magnetized if necessary. Magnet composition The magnet composition is governed by the composition of primary phase-forming master alloy, the composition of grain boundary phase-forming master alloy, and the mixing ratio of the two alloys. The present invention requires that the primary phase-forming master alloy has the above-defined structure and the grain boundary-forming master alloy has the above-defined composition although it is preferred that the magnet as sintered have a composition consisting essentially of 27 to 38% by weight of R, 0.5 to 4.5% by weight of B, 0.03 to 0.5%, especially 0.05 to 0.3% by weight of M, and 51 to 72% by weight of T. Residual magnetic flux density increases as the R content decreases. However, a too low R content would allow α-Fe and other iron rich phases to precipitate to adversely affect pulverization and magnetic properties. Also since a reduced proportion of an R rich phase renders sintering difficult, the sintered density becomes low and the residual magnetic flux density is no longer improved. In contrast, even when the R content is as low as 27% by weight, the present invention is successful in increasing the sintered density and eliminating substantial precipitation of an α-Fe phase. If the R content is below 27% by weight, however, it would be difficult to produce a useful magnet. A too high R content would adversely affect residual magnetic flux density. A too low boron content would adversely affect coercivity whereas a too high boron content would adversely affect residual magnetic flux density. EXAMPLE Examples of the present invention are given below by way of illustration and not by way of limitation. Example 1 By cooling an alloy melt having the composition consisting essentially of 28% by weight Nd, 1.2% by weight Dy, 1.2% by weight B and the balance of Fe by a single roll technique in an Ar gas atmosphere, there were produced a series of primary phase-forming master alloys in thin ribbon form which are reported as Nos. 1-1 to 1-7 in Table 1. Table 1 also reports the thickness of primary phase-forming master alloy in the cooling direction and the peripheral speed of the chill roll. The chill roll used was a copper roll. For comparison purposes, an alloy melt having the composition of 26.3% Nd, 1.2% Dy, 1.2% B and the balance of Fe, in % by weight, was cooled in an argon atmosphere by a single roll technique, obtaining primary phase-forming master alloys in thin ribbon form which are reported as Nos. 1-8 and 1-9 in Table 1. Table 1 also reports the thickness of these primary phase-forming master alloys in the cooling direction and the peripheral speed of the chill roll. The chill roll used was a copper roll. Each master alloy was cut to expose a section including the cooling direction. The section was then polished for imaging under an electron microscope to take a reflection electron image. FIG. 4 is a photograph of sample No. 1-3 which indicates the presence of columnar crystal grains having a major axis substantially aligned with the cooling direction or the thickness direction of the thin ribbon. In some samples, isometric grains were also observed. For each master alloy, the mean grain size was determined by measuring the diameter of one hundred columnar grains across this section. Using scanning electron microscope/energy dispersive X-ray spectroscopy (SEM-EDX), each master alloy was examined for the presence of an -Fe phase and isometric grains. The results are also reported in Table 1. The amount of R rich phase of sample Nos. 1-2 - 1-4 are 1 to 10 vol%, however in example Nos. 1-8 and 1-9, R rich phase substantially did not exist. Each primary phase-forming master alloy was crushed into a primary phase-forming master alloy powder having a mean particle size of 15 µ m. Separately, for sample Nos. 1-1 to 1-7, an alloy having the composition consisting essentially of 38% by weight Nd, 1.2% by weight Dy, 15% by weight Co and the balance of Fe was melted by high-frequency induction in an argon atmosphere and cooled into an alloy ingot. This alloy ingot contained R 3 (Co,Fe), R(Co,Fe) 5 , R(Co,Fe) 3 , R(Co,Fe) 2 , and R 2 (Co,Fe) 17 phases and had a mean grain size of 25 µ m. The alloy ingot was crushed into a grain boundary phase-forming master alloy powder having a mean particle size of 15 µ m. For sample Nos. 1-8 and 1-9, a grain boundary phase-forming master alloy powder was prepared by the same procedure as above except that the starting alloy contained 43.8% by weight of Nd. By mixing 80 parts by weight of the primary phase-forming master alloy powder and 20 parts by weight of the grain boundary phase-forming master alloy powder, there was obtained a mixture of the composition consisting essentially of 28.8% by weight Nd, 1.2% by weight Dy, 1% by weight B, 3% by weight Co, and the balance of Fe. The mixture was subject to hydrogen occlusion treatment under the following conditions and then to mechanical pulverization without hydrogen release treatment. Hydrogen occlusion treatment conditions Mixture temperature 400°C Treating time 1 hour Treating atmosphere hydrogen atmosphere of 0.5 atm. A jet mill configured as shown in FIG. 2 was used for mechanical pulverization. The mixture was milled until the respective alloy powders reached a mean particle size of 3.5 µ m. The microparticulate mixture was compacted under a pressure of 1.5 t/cm 2 in a magnetic field of 15 kOe. The compact was sintered in vacuum at 1,075°C for 4 hours and then quenched. The sintered body was subjected to two-stage aging in an argon atmosphere before a magnet was obtained. The first stage of aging was at 850°C for 1 hour and the second stage of aging was at 520°C for 1 hour. The magnet was measured for magnetic properties which are reported in Table 1. It is evident from Table 1 that high performance magnets are obtained when the primary phase-forming master alloy contains columnar grains having a mean grain size of 3 to 50 µ m. Those primary phase-forming master alloys substantially free of an R rich phase have relatively poor magnetic properties (Nos. 1-8 and 1-9). Example 2 Sample Nos. 2-1 to 2-15 (invention) Grain boundary-forming master alloys were prepared by using Nd, Fe, Co, Sn, Ga and In components, all of 99.9% purity, and arc melting the components in an argon atmosphere. Separately, primary phase-forming master alloys were prepared by using Nd, Dy, Fe, Co, Al, Si, Cu, ferroboron, Fe-Nb, Fe-W, Fe-V, and Fe-Mo components, all of 99.9% purity, and melting the components in an argon atmosphere by high-frequency induction heating. The compositions of the master alloys are shown in Table 2. Each of the master alloys was independently crushed by a jaw crusher and brown mill in a nitrogen atmosphere. A crude powder of grain boundary-forming master alloy and a crude powder of primary phase-forming master alloy were mixed in a nitrogen atmosphere. The mixing proportion (weight ratio) and the composition of the resulting mixture (which conforms to the magnet's composition) are shown in Table 2. Next, the mixture was finely comminuted to a particle size of 3 to 5 µ m by means of a jet mill using high pressure nitrogen gas jets. The microparticulate mixture was compacted under a pressure of 1.5 t/cm 2 in a magnetic field of 12 kOe. The compact was sintered in vacuum at 1,080°C for 4 hours and then quenched. The sintered body was subjected to two-stage aging in an argon atmosphere. The first stage of aging was at 850°C for 1 hour and followed by cooling at a rate of 15°C/min. The second stage of aging was at 620°C for 1 hour and followed by cooling at a rate of 15°C/min. At the end of aging, the sintered body was magnetized, yielding a magnet sample. Each magnet sample was measured for magnetic properties including coercivity Hcj, maximum energy product (BH)max, and dHcj/dT in the temperature range between 25°C and 180°C using a BH tracer and vibrating sample magnetometer (VSM). Separately, each sample was processed so as to have a permiance coefficient of 2, magnetized in a magnetic field of 50 kOe, kept in a constant temperature tank for 2 hours, and cooled down to room temperature. Using a flux meter, the sample was measured for irreversible demagnetization. The temperature at which the irreversible demagnetization reached 5%, T(5%), was determined. The results are shown in Table 2. Example 3 (comparison) Sample Nos. 3-1 to 3-5 (comparison) Magnet-forming master alloys of the composition shown in Table 3 were prepared by the same procedure as used for the primary phase-forming master alloy of the inventive samples. Like the inventive samples, the magnet-forming master alloys were crushed, finely milled, compacted, sintered, aged, and magnetized, obtaining magnet samples. These samples were similarly measured for magnetic properties. The results are shown in Table 3. A comparison of sample No. 2-1 with No. 3-3, a comparison of sample No. 2-2 with No. 3-2, and a comparison of sample Nos. 2-3 and 2-4 with No. 3-4 reveal that the inventive samples have at least equal thermal stability even when their Sn content is one-half of that of the comparative samples and better magnetic properties are obtained due to the reduced Sn content. A comparison of sample No. 2-1 with No. 3-2 reveals that for the same Sn content, the inventive sample is improved in thermal stability and magnetic properties. A comparison of sample No. 2-2 with No. 2-5 reveals that thermal stability and magnetic properties are improved as the composition of the grain boundary-forming master alloy is closer to R 6 T' 13 M. It is understood that sample No. 2-2 uses a grain boundary-forming master alloy of the composition: 50.5Nd-42.5Fe-7.0Sn (% by weight) which corresponds to Nd 6 Fe 13 sn as expressed in atomic ratio. A comparison of sample No. 2-6 with No. 3-3 reveals that for the same Sn content, the inventive sample is effective for minimizing a loss of magnetic properties. Sample Nos. 2-7 and 2-8 show that addition of Ga and In is equally effective. The grain boundary-forming master alloys used in the inventive samples shown in Table 2 contained R 6 T' 13 M, RT' 2 , RT' 3 , RT' 7 , and R 5 T' 13 phases and had a mean grain size of 20 µ m. Identification of phases and measurement of a grain size were carried out by SEM-EDX after polishing a section of the alloy. Example 4 Sample No. 4-1 (invention) A primary phase-forming master alloy was prepared by a single roll process. The chill roll used was a copper roll which was rotated at a circumferential speed of 2 m/s. The resulting alloy had a thin ribbon form of 0.3 mm thick and 15 mm wide. The composition of the primary phase-forming master alloy is shown in Table 4. The master alloy was cut to expose a section including the cooling direction. The section was then polished for imaging under an electron microscope to take a reflection electron image. The photograph indicates the presence of columnar crystal grains having a major axis substantially aligned with the cooling direction or the thickness direction of the thin ribbon. By measuring the diameter of one hundred columnar grains across this section, the mean grain size was determined to be 10 µ m. The presence of -Fe phase was not observed in this master alloy. This master alloy was crushed as in Example 2. A grain boundary-forming master alloy was prepared and crushed in the same manner as in Example 2. The composition of the grain boundary phase-forming master alloy is shown in Table 4. The crude powder of grain boundary-forming master alloy and the crude powder of primary phase-forming master alloy were mixed in a nitrogen atmosphere. The mixing proportion (weight ratio) is shown in Table 4. The mixture was subject to hydrogen occlusion treatment under the following conditions and then to mechanical pulverization without hydrogen release treatment. Hydrogen occlusion treatment conditions Mixture temperature 400°C Treating time 1 hour Treating atmosphere hydrogen atmosphere of 0.5 atm. A jet mill configured as shown in FIG. 2 was used for mechanical pulverization. The mixture was milled until the respective alloy powders reached a mean particle size of 3.5 µ m. The subsequent steps were the same as in Example 2. The resulting magnet sample was similarly measured for magnetic properties. The results are shown in Table 4. Sample No. 4-2 (invention) A magnet sample was manufactured by the same procedure as sample No. 4-1 except that a grain boundary-forming master alloy was prepared by a single roll process under the same conditions as the primary phase-forming master alloy of sample No. 4-1. The grain boundary-forming master alloy had a ribbon form of 0.3 mm thick and 15 mm wide. The resulting magnet sample was similarly measured for magnetic properties. The results are shown in Table 4. Sample No. 4-3 (invention) A magnet sample was manufactured by the same procedure as sample No. 4-2 except that upon preparation of a grain boundary-forming master alloy by a single roll process, the circumferential speed of the chill roll was changed to 30 m/s. The resulting magnet sample was similarly measured for magnetic properties. The results are shown in Table 4. Sample Nos. 4-4 to 4-5 (comparison) Magnet-forming master alloys of the composition shown in Table 4 were prepared by a melting or single roll process. The single roll process used the same conditions as inventive sample No. 4-1. Like the inventive samples, the magnet-forming master alloys were crushed, finely milled, compacted, sintered, aged, and magnetized, obtaining magnet samples. These samples were similarly measured for magnetic properties. The results are shown in Table 4. The grain boundary-forming master alloys used in the inventive sample Nos. 4-1 and 4-2 contained R 6 T' 13 M, RT' 2 , RT' 3 , RT' 7 , and R 5 T' 13 phases. Sample Nos. 4-1 and 4-2 had a mean grain size of 25 µ m and 10 µ m, respectively. The grain boundary-forming master alloy used in the inventive sample No. 4-3 was amorphous. As is evident from Table 4, very high values of (BH) max are obtained when primary phase-forming master alloys containing columnar grains having a mean grain size of 3 to 50 µ m are used. Thermal stability and magnetic properties are further improved when grain boundary phase-forming master alloys containing grains having a mean grain size of up to 20 µ m are used as in sample Nos. 4-2 and 4-3. It was found that when Fe in the grain boundary-forming master alloy was partially replaced by Ni, the results were equivalent to those of the foregoing examples. When the grain boundary-forming master alloy was annealed at 700°C for 20 hours, the proportion of R 6 T' 13 M phase increased. A magnet sample using this master alloy had magnetic properties and thermal stability comparable to those of the inventive samples. Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims: A method for preparing a permanent magnet which contains R, B, T and M as main ingredients wherein R is at least one element selected from the group consisting of yttrium and rare earth elements, T is iron or a mixture of iron and cobalt, B is boron and M is at least one element selected from the group consisting of tin, indium and gallium, wherein M contains 30 to 100% by weight of tin, and which has a primary phase consisting essentially of R 2 T 14 B, said method comprising the steps of compacting a mixture of a primary phase-forming master alloy and a grain boundary-forming master alloy both in powder form and sintering the compact, wherein said primary phase-forming master alloy has a phase consisting essentially of R 2 T 14 B, and grain boundaries composed mainly of an R rich phase having a higher R content than R 2 T 14 B, wherein said grain boundary-forming master alloy has an R 6 T' 23 M phase and said grain boundary-forming master alloy contains 40 to 65% by weight of R, 30 to 60% by weight of T' and 1 to 12% by weight of M, wherein T' is at least one element selected from the group consisting of iron and cobalt. The method of claim 1 wherein said mixture contains 0.2 to 10% by weight of said grain boundary-forming master alloy. The method of claim 1 or Claim 2, wherein the primary phase of said primary phase-forming master alloy contains columnar crystal grains having a mean grain size of 3 to 50 µm. The method of claim 1, 2 or 3 wherein the permanent magnet consists essentially of 27 to 38% by weight of R, 0.5 to 4.5% by weight of B, 0.03 to 0.5% by weight of M, and 51 to 72% by weight of T. The method of claim 1, 2, 3 or 4 wherein the permanent magnet contains an R 6 T' 13 M phase in the grain boundary. The method of any one of claims 1 to 5 wherein said primary phase-forming master alloy is produced by cooling an alloy melt from one direction or two opposite directions. The method of claim 6 wherein the alloy melt is cooled by a single roll, twin roll or rotary disk process. The method of claim 6 or 7 wherein said primary phase-forming master alloy as cooled has a thickness of 0.1 to 2 mm in the cooling direction. The method of any one of claims 1 to 8 wherein said primary phase-forming master alloy is substantially free of an α-Fe phase. The method of claims 1 to 9 wherein said grain boundary phase-forming master alloy contains grains having a mean grain size of up to 20 µm. The method of any one of claims 1 to 10 wherein said grain boundary phase-forming master alloy is produced by cooling an alloy melt from one direction or two opposite directions. The method of claim 11 wherein the alloy melt is cooled by a single roll, twin roll or rotary disk process. The method of claim 11 or 12 wherein said grain boundary phase-forming master alloy as cooled has a thickness of 0.1 to 2 mm in the cooling direction. The method of any one of claims 1 to 13 wherein said primary phase-forming master alloy in powder form is produced by causing the alloy to occlude hydrogen and pulverizing the alloy by a jet mill. The method of any one of claims 1 to 14 wherein said grain boundary phase-forming master alloy in powder form is produced by causing the alloy to occlude hydrogen and pulverizing the alloy by a jet mill. The method of claim 14 or 15 wherein the alloy is heated to a temperature of 300 to 600°C, subjected to hydrogen occlusion treatment, and then pulverized without hydrogen release. The method of claim 14 or 15 wherein the hydrogen occlusion is followed by hydrogen release. The method of claim 3, wherein said columnar crystal grains have a major axis length to width ratio of 2/1 to 50/1.

Inventors: FUJITO SHINYA, HASHIMOTO SHINYA, TAKEBUCHI KATASHI, YAJIMA KOICHI, FUJITO, SHINYA, HASHIMOTO, SHINYA, TAKEBUCHI, KATASHI, YAJIMA, KOICHI, Fujito, Shinya c/o TDK Corporation, Hashimoto, Shinya c/o TDK Corporation, Takebuchi, Katashi c/o TDK Corporation, Yajima, Koichi c/o TDK Corporation
Applicants: TDK CORP, TDK CORPORATION
Application year: 1994
Main classification: H01F1
IPC codes: H01F1